Re: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM_C

Josh Poimboeuf
2021-10-16T21:16:51Z

On Sat, Oct 16, 2021 at 12:17:40AM +0200, Thomas Gleixner wrote:
> For actually callable functions, by some definition of callable,
> e.g. the clear_page_*() variants a proper attribute would be definitely
> preferred.

See my last email, clear_page_*() has nothing to do with CFI in the
first place.

> That attribute should tell the compiler that the function is using the
> register arguments correctly but is not suitable for direct invocation
> because it clobbers registers.
>
> So the compiler can just refuse to call such a function if used directly
> without an inline asm wrapper which describes the clobbers, right?
>
> But thinking more about clobbers. The only "annotation" of clobbers we
> have today are the clobbers in the inline asm, which is fragile too.
>
> Something like
>
> __attribute__ ((clobbers ("rcx", "rax")))
>
> might be useful by itself because it allows validation of the clobbers
> in the inline asm wrappers and also allows a analysis tool to look at
> the ASM code and check whether the above list is correct.
>
> Hmm?

Functions are allowed to clobber rcx and rax anyway.

The clear_page_*() functions follow the C ABI, like (almost) every other
asm function in the kernel. I think there's a misunderstanding here, as
most of this doesn't have anything to do with CFI anyway.

--
Josh